A desire to minimise the dependence on coal, oil and gas and to reduce the CO2 emission has intensified the research in areas concerned with the exploitation of renewable biomasses such as wood, straw and plant deposits into oil-based products such as petrol, diesel, chemicals and plastics through the formation of syngas.
Numerous industrial and agricultural processes e.g. municipality operations, food and feed processing and forestry generate biomasses, waste and by-products containing polymeric sugars e.g. in the form of starch, cellulose and hemicellulose. Agribusiness and chemical industries as well as public organisations have considerable interest in developing processes for converting such biomasses into materials of a higher value. However, the majority of processes known today have not yet reached large-scale commercial practice due to their high production cost and high energy demand and thus inherent uncertain economic feasibility.
Besides being important as food and feed, carbohydrates from biomass through syn-gas production can be used as feedstock for a number of industrial processes producing products valuable in the production of oil-based products such as petrol, diesel, chemicals and plastics.
It is therefore evident that if low-cost and abundant resources of carbohydrates can be processed into e.g. syngas at a relatively low energy consumption and thereby can be made available for industrial processing it may have a substantial economic potential.
Biomasses such as wood, straw and plant deposits can be converted into oil-based products such as petrol, diesel, chemicals and plastics through the formation of syngas by use of a gasifier. Syngas consists of CO, CO2, H2, N2, CH4, H2O and impurities such as H2S and tars. Gasification is a well-known technology where hydrocarbon bonds are broken down to produce syngas, under the addition of oxygen and steam, preferably under high pressures and temperatures. A gasifier differs from a combustor in that the amount of air or oxygen available inside the gasifier is carefully controlled so that only a relatively small portion of the fuel burns completely. The oxygen level controls that the hydrocarbons do not combust into CO2, but only oxidises partially.
A major obstacle to obtain full benefit from gasification of biomass compared to coal is how to pre-treat the biomass in a way, that makes it suitable to be economically gasified (ECN, 2004). Syngas production from coal gasification has been a commercial available technology for more than 50 years. Coal is relatively easy to grind and feed into a pressurised gasifier, biomass in general however is often very troublesome to grind and fed into a pressurised gasifier. This is due to the fact that most biomass is very inhomogeneous compared to coal, and hence it is difficult to grind it down to a relative homogeneous particle size required for entrained flow gasification where the residence time is very short—in the order of few seconds. Furthermore it is very troublesome to pressurise a solid material, with a very uneven and rather large particle size.
Traditionally, pretreatment of biomasses to be fed into a gasifier have been performed using methods such as direct pulverisation, combustion and pulverisation, production of charred sludge through flash-pyrolysis or production of gaseous fuels through low-temperature fluid-bed gasification which are all combined with different disadvantages concerning energy costs and/or economic costs.
The present invention relates to a method to liquefy the biomass in a way that it becomes a “homogenised liquid” with a rather small particle size and still have a rather high dry matter content (above 20%). The homogenised liquid can economically be pressurised using commercial pumps, and as a consequence it will be possible to feed a pressurised entrained flow gasifier with biomass as bagasse, straw etc.
Depending on the type of biomass intended to be used, several different “pre-treatment” options exist in order to liquefy the biomass.
Pre-treatment is required if a subsequent hydrolysis (e.g. enzymatic hydrolysis) of the polysaccharides requires the break down of an otherwise protecting structure (e.g. lignin) of the plant materials. Several pre-treatment techniques are known. For cereals and grains, this pre-treatment may be in the form of a simple dry milling in order to render the surfaces accessible, but for lignocellulosic biomasses thermal and/or chemical processes are needed as well. A polysaccharide containing biomass consisting of e.g. refined starch does not require said pre-treatment methods prior to enzymatic processing. Pretreatment-processes may be based on acidic hydrolysis, steam explosion, oxidation, extraction with alkali or ethanol etc. A common feature of the pre-treatment techniques is that combined with the action of possible added reactants they take advantage of the softening and loosening of plant materials that occurs at temperatures above 100° C.
Apart from starch the three major constituents in plant biomass are cellulose, hemicellulose and lignin, which are commonly referred to by the generic term lignocellulose. Polysaccharide containing biomasses as a generic term include both starch and lignocellulosic biomasses.
Cellulose, hemicellulose and lignin are present in varying amounts in different plants and in the different parts of the plant and they are intimately associated to form the structural framework of the plant.
Cellulose is a homopolysaccharide composed entirely of D-glucose linked together by β-1,4-glucosidic bonds and with a degree of polymerisation up to 10,000. The linear structure of cellulose enables the formation of both intra- and intermolecular hydrogen bonds, which results in the aggregation of cellulose chains into micro fibrils. Regions within the micro fibrils with high order are termed crystalline and less ordered regions are termed amorphous. The micro fibrils assemble into fibrils, which then form the cellulose fibres. The partly crystalline structure of cellulose along with the microfibrillar arrangement, gives cellulose high tensile strength, it makes cellulose insoluble in most solvents, and it is partly responsible for the resistance of cellulose against microbial degradation, i.e. enzymatic hydrolysis.
Hemicellulose is a complex heterogeneous polysaccharide composed of a number of monomer residues: D-glucose, D-galactose, D-mannose, D-xylose, L-arabinose, D-glucuronic acid and 4-O-methyl-D-glucuronic acid, Hemicellulose has a degree of polymerisation below 200, has side chains and may be acetylated. In softwood like fir, pine and spruce, galactoglucomannan and arabino-4-O-methyl-glucuronoxylan are the major hemicellulose fractions. In hardwood like birch, poplar, aspen or oak, 4-O-acetyl-4-methyl-glucuronoxylan and glucomannan are the main constituents of hemicellulose. Grasses like rice, wheat, oat and switch grass have hemicellulose composed of mainly glucuronoarabinoxylan.
Lignin is a complex network formed by polymerisation of phenyl propane units and it constitutes the most abundant non-polysaccharide fraction in lignocellulose. The three monomers in lignin are p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, and they are most frequently joined through arylglyceryl-β-aryl ether bonds. Lignin is linked to hemicellulose and embeds the carbohydrates thereby offering protection against microbial and chemical degradation.
Starch is the most widespread storage carbohydrate in plants and occurs in the form of granules, which differ markedly in size and physical characteristics from species to species. Starch granules are generally quite resistant to penetration by both water and hydrolytic enzymes due to the formation of hydrogen bonds within the same molecule and with other neighbouring molecules. However, these inter- and intra-hydrogen bonds can become weak as the temperature of the suspension is raised. When an aqueous suspension of starch is heated, the hydrogen bonds weaken, water is absorbed, and the starch granules swell. This process is commonly called gelatinization because the solution formed has a gelatinous, highly viscous consistency. Chemically, starch is a natural polymer of glucose, which is generally insoluble but dispersible in water at room temperature and made up of a repeating unit similar to that of cellulose and linked together by α-1,4 and α-1,6glucosidic bonds, as opposed to the β-1,4glucosidic bonds for cellulose. The units form either a linear chain component, called amylose, or a branched chain component, called amylopectin. Most plant seeds, grains and tubers contain about 20-25% amylose. But some, like e.g. pea starch have 60% amylose and certain species of corn have 80% amylose. Waxy varieties of grains, such as rice, are low in amylose.
Following the pre-treatment, the next step in utilisation of polysaccharide containing biomasses for production of syngas is hydrolysis of the liberated starch, cellulose and hemicellulose into polymeres and oligomeres.
This can be obtained with different enzymes with different modes of action. The enzymes can be added externally or microorganisms growing on the biomass may provide them.
Cellulose is hydrolysed into glucose by the carbohydrolytic cellulases. The prevalent understanding of the cellulolytic system divides the cellulases into three classes; exo-1,4-β-D-glucanases or cellobiohydrolases (CBH) (EC 3.2.1.91), which cleave off cellobiose units from the ends of cellulose chains; endo-1,4-β-D-glucanases (EG) (EC 3.2.1.4), which hydrolyse internal β-1,4-glucosidic bonds randomly in the cellulose chain; 1,4-β-D-glucosidase (EC 3.2.1.21), which hydrolyses cellobiose to glucose and also cleaves of glucose units from cellooligosaccharides.
The different sugars in hemicellulose are liberated by the hemicellulases. The hemicellulytic system is more complex than the cellulolytic system due to the heterologous nature of hemicellulose. The system involves among others endo-1,4-β-D-xylanases (EC 3.2.1.8), which hydrolyse internal bonds in the xylan chain; 1,4-β-D-xylosidases (EC 3.2.1.37), which attack xylooligosaccharides from the non-reducing end and liberate xylose; endo-1,4-β-D-mannanases (EC 3.2.1.78), which cleave internal bonds; 1,4-β-D-mannosidases (EC 3.2.1.125), which cleave mannooligosaccharides to mannose. The side groups are removed by a number of enzymes; α-D-galactosidases (EC 3.2.1.22), α-L-arabinofuranosidases (EC 3.2.1.55), α-D-glucuronidases (EC 3.2.1.139), cinnamoyl esterases (EC 3.1.1.-), acetyl xylan esterases (EC 3.1.1.6) and feruloyl esterases (EC 3.1.1.73).
The most important enzymes for use in starch hydrolysis are alpha-amylases (1,4-α-D-glucan glucanohydrolases, (EC 3.2.1.1). These are endo-acting hydrolases which cleave 1,4-α-D-glucosidic bonds and can bypass but cannot hydrolyse 1,6-alpha-D-glucosidic branchpoints. However, also exo-acting glycoamylases such as beta-amylase (EC 3.2.1.2) and pullulanase (EC 3.2.1.41) can be used for starch hydrolysis. The result of starch hydrolysis is primarily glucose, maltose, maltotriose, α-dextrin and varying amounts of oligosaccharides.
Enzymatic liquefaction and hydrolysis of biomass has previously been described. However, in case of pre-treated lignocellulosic biomasses only material consisting of fibres and particles with an average size below 1 inch (25.4 mm) and furthermore having a relatively low dry matter content, i.e. below 20% (w/w), have successfully been liquefied by such a method.
US2004/0262220 describes a method for anaerobic digestion of biomasses with the purpose of producing biogas. The method is unsuitable for the production of syngas as it is anaerobic and is furthermore designed to produce biogas, which in contrast to syngas consists mainly of CH4 and CO2. The pre-treatment of the biomass comprises thermal pre-heating and hydrolysis, however, in contrast to the present invention, thermal pre-heating of the biomass does not surpass 100° C. and hydrolysis results predominantly in mono saccharides which are suitable for fermentation.
U.S. Pat. No. 5,424,417 relates to a method of prehydrolysis of lignocellulose with the purpose of obtaining biomass suitable for further fermentation. The method is based on thermal treatment of 120-240° C. combined with the addition of alkali or acid in a flow-through system and includes a step removing solids. Such a flow-through system is not beneficial for the pre-treatment of biomass for syngas production as the entire slurry is to be pumped into a gasifier.
U.S. Pat. No. 4,645,541 concerns a multi-step method of producing microcrystalline cellulose and glucose from lignocellulosic material. The method comprises thermal pre-treatment of 185-240° C., explosively expelling of the material, extraction of lignin using an organic solvent, filtering and separating the material into cellulose and hemicellulose fractions. Since the purpose of this treatment is to separate lignin from cellulose and hemicellulose, the pre-treatment is much more laborious and energy consuming than what is needed for syngas production and results in particles of 1-10 microns already after the explosive step.
U.S. Pat. No. 4,916,242 concerns a process for thermally and chemically treating lignocellulose-containing biomass with the purpose of producing furfural. The method comprises heating the biomass in a cooking liquor containing pentoses. The liquor is distilled in a separate production plant producing the furfural, and the thermally treated biomass is discharged and hence not used for further production of e.g. syngas.
Gasification is a well-known technology where carbon-containing material is fed into a vessel in the addition of oxygen and steam. Traditionally, pulverised coal has been preferred as hydrocarbon feedstock. A reaction developing excessive heat takes place, the carbon bonds are broken and syngas is produced. Depending on the further use of the syngas, it may need to be cleaned from particles such as sulphur, alkali and mercury. If the syngas is to be burned in a gas turbine the cleaning of the gas is less important than if it is to be used for production synthetic product such as FT diesel, Petrol or methanol. Gasification can be more flexible, efficient and environmentally friendly compared to direct combustion. Gasifiers are also used in Integrated Gasification Combined Cycle (IGCC) power plants allowing a higher efficiency in electricity production in contrast to ordinary coal boilers and steam boilers.
Typically, coal, crude oil, high sulphur fuel oil, petroleum and other refinery residuals has been the preferred raw material for gasification. In a modern gasifier coal is subjected to hot steam and carefully controlled amounts of air or oxygen, under high temperatures and pressures. These conditions cause the carbon molecules in coal to break apart, starting chemical reactions that produce a mixture of hydrogen, carbon monoxide and other gaseous compounds (RRI, 2005). The most important period for coal gasification was during the 1980s and 1990s. Driven by environmental concerns over the burning of coal, gasification became known as a clean way to generate electric power. The first coal gasification electric power plants are now operating commercially (RRI, 2005).
However, substitution of the dwindling fossil fuels by renewable energy and carbon resources such as lignocellulosic biomass sources will be advantageous in the future. Herbaceous by-products from agriculture, mainly cereal straw and strawlike residues are cheap renewables. Wood is a relatively clean fuel and traditional technologies for wood combustion and gasification are well developed whereas the use of herbaceous biomass are more complex and not well developed (Henrich and Dinjus, 2002). Biomass differs from coal in many respects. The most relevant differences refer to ash behaviour, feeding and pressurising properties.
Biomass is generally very variable in respect of structure, content of water, content of alkali and particle size. This complicates the processing of biomass, especially if it is to be treated under pressurised conditions. Therefore, one of the greatest challenges to optimal benefit from gasification is to pre-treat the biomass in a way which makes it suitable to be fed into the gasifier. In comparison with other potential biomasses for syngas production, enzymatically liquefied lignocellulosic biomass obtained by the present method has the great advantage that it is already partially processed and exist in a pumpable, liquid form with suspended particles of limited size (typical below 1 mm).
The calorific value of the biomass is very dependent on the water content, which is therefore a limiting factor for the quality of the syngas developed. A higher combustion temperature results in a syngas containing a lower amount of tar products and methane. On the other hand, a high water content represses the soot development and is thermodynamically favourable in respect to limit CH4 slip exit the gasifier Biomasses such as straw have a high content of alkali such as potassium and sodium.
During the gasification some of this is released in the gas phase and will subsequently condense during the cooling of the gas and result in problems related to fouling and corrosion. In oxygen based entrained flow gasifiers a temperature of 2500° C. can be reached in the combustion zone whereby the majority of the minerals in the biomass will fuse into slag and will therefore not be available for re-delivery into the environment.
There are several gasification methods available, however, gasification using pure oxygen at high temperature and high pressure results in the highest efficiency and best quality syngas (Hamelinck et al., 2003). The central component is the gasifier regulating the oxygen flow such that the hydrocarbons do not combust completely into CO2 but only oxidises partially.
Gasification processes can be divided into four major classes that involve operating pressures up to around 400 bar:    Moving bed/Moving fixed bed    Fluidised bed    Entrained flow    Supercritical Gasification
Moving bed gasifiers have a considerable distribution but are mainly suited for solid fuels. The gasifier consists of introducing coarse solids at the top of the gasifier while the oxidant gas and steam are introduced at the bottom of the gasifier. The thermal efficiency of the gasifier is high but since the gasifier produces tars and oils, the gas clean up is complex. Biomass such as straw and liquefied biomass is not suitable for this kind of gasifier.
Fluidised bed gasifiers burn the hydrocarbon feedstock in a bed of heated particles suspended in flowing air. At sufficiently high air velocity, the bed acts as a fluid resulting in rapid mixing of the particles. The fluidising action promotes complete combustion at relatively low temperatures (760-1040° C.) and provides a means to transfer combustion heat efficiently from the bed to steam tubes. The use of sulphur-absorbent chemicals such as limestone or dolomite is indispensable. However, since the absorbents may react with the alkali compounds of the biomass to form a low melting suspension with the risk of plugging the gasifier, removal of alkali from the biomass prior to feeding into the gasifier will supposedly be advantageous. Due to the low temperatures, the residues from fluidised bed gasifiers are less inert than ashes produced in a moving bed gasifier and may require more attention to their disposal in an environmentally secure repository.
Entrained flow gasifiers are the most wide spread gasifiers for commercial purposes. Such gasifiers are usually fed with a coal and water slurry but can also operate on dry coal by use of pneumatics and/or lock hoppers. The biggest advantage of using an entrained flow reactor is the high exit temperatures resulting in a syngas with a very low content of tar and methane. The entrained flow gasifiers is also characterised by having a rather short retention time of say 1-2 seconds. The advantages of such a short retention time, is that very high amounts of coals/biomass can be converted at a relatively small gasifier volume, which is the driving force for choosing entrained flow gasifiers for large scale commercial applications. The disadvantages is on the other hand that the coal/biomass, has to be grinded into very small particles in order to complete the chemical reactions, before it leaves the gasifier. The entrained flow gasifier is very suitable for the production of hydrogen and syngas products. Combustion in this kind of gasifier may result in slagging residuals depending on the potential ash content of the biomass. In a slagging gasifier, the ash forming components melt in the gasifier, flow down the walls of the reactor and finally leave the reactor as a liquid slag (ECN, 2004).
Coal based conventional and well-tested systems require a previous pulverisation of the coal resulting in particles of 40-100 μm. Similar mechanical pre-treatment of biomass such as wood uses up to 0.08 kW electricity/kW wood corresponding to approximately 20% primary energy (ECN, 2004), which is an unacceptable high value.
Different pre-treatment of biomasses to be converted in an entrained flow gasifier has been tested including direct wood pulverisation production of brittle solid by torrefaction and subsequent pulverisation, production of oil/char-slurry by flash pyrolysis and production of gaseous fuel by low temperature fluidised bed gasification (ECN, 2004). In general, coal-fired entrained flow gasifiers are operated on coal powders with a size of typically 50-100 μm. This ensures complete conversion. Since biomass is much more reactive than coal, it is expected that size demands for biomass are less stringent. The ECN (2004) report states, that biomass particles can be as large as 1 mm as far as complete conversion is concerned.
Fast or flash pyrolysis is a relatively simple method that converts about half of lignocellulosic biomass or even more into a pyrolysis liquids. The brittle pyrolysis char is pulverised and suspended in oil to produce pumpable slurry (Henrich and Dinjus, 2002). However, fast pyrolysis requires that the biomass is dried and chopped and further reduced in size using a hammer mill to ensure a fast heatup during pyrolysis. Most reactors suitable for fast pyrolysis use a solid heat carrier such as sand. The fluidisation is obtained by mechanical means. The biofuel particles are mixed with and excess of hot sand above 500° C. and transported in co-current flow with low axial and good radial mixing (Henrich and Dinjus, 2002).
Pre-treatment of biomass at low-temperature pyrolysis or out-burning implies that salts, which may course problems in the gasifier, remain in the material. Alternatively, the salts, which are often useful fertilisers, are bound in residuals and thereby the possibility to re-circulate them into the environment is lost. Furthermore, pre-treatment based on pyrolysis is not a very attractive method, as the efficiency is low.
Feeding of biomass into an entrained gasifier may be done using lock hoppers or piston feeders. In the lock hopper system, the lock hopper is filled with biomass at atmospheric pressure, pressurised to 4000 psi using an inert gas and the solids are fed to the reactor with the help of a rotary feeder (LLC, 2006), screw feeder or by pneumatic transport.
Piston feeding is an alternative for the lock hopper system and has the advantage of little volume and low inert gas consumption. This method has been tested for torrified wood chips (ECN, 2004). This piston feeder consisted of an atmospheric supply bunker, a piston feeder and a pressurised tank Subsequently the biomass can be fed by screw into the gasifier. The experiment showed that if a piston feeder replaced the lock hopper, the inert gas consumption was reduced and the energy penalty was lower than 3% for 1 mm solids.
However, for liquefied biomasses, pressurising systems can be replaced by slurry pumps that are state-of-the-art. The liquid fuel subsequently is atomised and fed to the burner similarly to solid fuel powder (ECN, 2004).
Supercritical Gasification is a process where the feedstock is heated and pressurised to a value higher than the critical point of water (221 bar and 375° C.). When the feedstock is sufficiently heated and pressurised the organic part decomposes into a mixture of hydrogen, methane, carbon monoxide and carbon dioxide. If the amount of water is relatively high compared to the amount of organic materials, all the CO will be converted into hydrogen and carbon dioxide due to the watergas shift reaction. Presently supercritical gasification is under development by i.e. Forschungszentrum Karlsruhe ( DE10210178). Due to the fact that the biomass needs to be pressurised up to around 300 bar, the biomass needs to be pumpable. So far this has been solved by fine chopping the biomass and diluting it with water, resulting in a biomass water slurry with a dry matter content of only around 10% based on weight. It will from an efficiency and economically point of view, be a huge advantage to increase the dry matter content to around 20-30%, which is possible with the present invention.